A mobile node (MN) is typically a host identified by its home IP address. When it moves to a new location, it has to notify its home agent. The home agent is a router located in the home subnet of the mobile node. The notification enables the home agent to route EP datagrams to the mobile node at its new location. The mobile node notifies its home agent of its new care of address by sending a Registration Request message. This registration mechanism is defined in the base Mobile IP protocol (MIP).
The base Mobile IP allows a correspondent host to transparently send datagrams to mobile nodes as it would to any other nodes. Datagrams addressed to the mobile node are always routed via the home agent in the home network. The mobile node keeps the home agent informed of its whereabouts. As the mobile node moves away from its home network, it may no longer be topologically close to its home agent. Route optimization [MIP-OPTIM] has been proposed to allow a host to send packets to the mobile node as the mobile node (MN) moves, without having to route the packets via the home agent each time. The mobile node provides its current address to a host (or correspondent node, CN) with which it is communicating as it moves. In MIP-OPTIM, a mobile node may notify the correspondent nodes of its care of address via the home agent.
As described in MIP-OPTIM, a mobile node may send a Binding Warning message to its home agent to request that the home agent inform (by sending Binding Update messages) the correspondent hosts of its new care-of address (COA). A mobile node may append this message (Binding Warning Extension) in the Registration Request message to the home agent. On reception of the Binding Warning Extension message, the home agent should send Binding Update messages to the correspondent hosts listed in the Binding Warning message, to notify the correspondent hosts of the mobile node's new care of address.
The correspondent host updates a cache of mobile node locations with this new address and tunnels datagrams (addressed to the mobile network device) to the current address of the mobile network device. However, not all existing host systems support tunneling. Thus, not all correspondent hosts are capable of tunneling IP datagrams as required by the route optimization mechanisms described in [MIP-OPTIM]. More importantly, disclosing the current location (or “care of address” (COA)) of the mobile node to correspondent hosts is not always desirable for security reasons. Nor is the overhead for the correspondent hosts of encapsulating datagrams to the mobile node ideal.